The present invention generally relates to printed circuit board apparatus, and more particularly relates to apparatus and methods for shielding EMI radiation generated by a printed circuit board during operation thereof.
Printed circuit boards used in various types of computers are mounted within a plastic housing structure. During operation of the circuit board electromagnetic interference (EMI) radiation is generated within the board, emanates therefrom, and must be substantially prevented from escaping outwardly through the housing structure. To provide the housing with the necessary EMI radiation shield the interior of the housing is typically coated with a metallic material which is appropriately brought into contact with a grounding portion of the electronics, such as the ground plane of the circuit board disposed within the housing. Other types of EMI shields positioned in a spaced apart location from the circuit board may be alternatively utilized.
A primary source of EMI radiation generated by a circuit board during its operation is commonly referred to as transverse electromagnetic mode (TEM) radiation. TEM radiation is created primarily by the alternating clock current transmitted through the circuit board traces, and the instantaneous current changes in the electronic board components such as chips, and is manifested as a generally wave-shaped mutual inductance disturbance between the spaced apart ground plane and power plane portions of the circuit board disposed within its dielectric substrate member.
The TEM wave effect can account for up to about seventy five percent of the total EMI interference generated by the operating circuit board, and creates EMI radiation that is discharged from the edge periphery of the board substrate in a generally parabolic pattern. TEM radiation discharged from the peripheral edge of the substrate in this manner strikes the remote EMI shield structure. A first portion of the received radiation is absorbed by the shield structure; a second portion of the received radiation is inwardly reflected by the shield structure; and the balance of the received radiation is conducted by the shield structure back to the circuit board ground plane, and then to the emitting component, via a grounding connection interconnected between the ground plane and the remote shield structure.
The remote EMI shield structure creates with the ground plane of the circuit board an RF loop around which the TEM radiation, and other types of EMI radiation, pass from and back to the radiation emitting component via the ground plane. The radiation shielding efficiency of this RF loop is generally proportional to its effective length. In the conventional circuit board/housing EMI shield structure generally described above, this loop is relatively correspondingly diminished. AAccordingly, the its overall EMI shielding efficiency is correspondingly diminished.
Another disadvantage associated with this conventional EMI shielding technique is that it typically requires a number of midboard mounting holes for board-to-shield grounding purposes. This requirement can add considerably to the complexity of routing conductive traces along the substrate since the traces must be diverted around the mounting holes.
As can readily be seen from the foregoing, it would be highly desirable to provide a housing-enclosed printed circuit board with improved apparatus and associated methods for electrically grounding the board and shielding the TEM radiation emitted by the board during the operation thereof. It is accordingly an object of the present invention to provide such improved apparatus and methods.